Apparatuses for voltage conversion of the type in question can be used in almost all electrical systems in which different system voltages are present and, according to function, a system voltage is transformed into another system voltage. An example of said systems is an onboard electrical system on a hybrid or electric vehicle. An onboard electrical system of this kind normally comprises two or more onboard electrical system branches that each have an onboard electrical system voltage, wherein the onboard electrical system voltages of different onboard electrical system branches have different voltage values. In this case, a first onboard electrical system branch comprises loads with low power consumption, such as a navigation appliance, which are supplied with a 12 volt onboard electrical system voltage. This first onboard electrical system branch is then referred to as the low voltage onboard electrical system branch by way of example. A second onboard electrical system branch comprises loads with high power consumption, such as an electric motor that is used as a starter for the internal combustion engine or as a drive for the vehicle. This second onboard electrical system branch is normally predominated by a comparatively high onboard electrical system voltage that is 100 volts or higher. This second onboard electrical system branch is then called the high voltage onboard electrical system branch by way of example. Between these two onboard electrical system branches, a flow of current often takes place in order to protect the second onboard electrical system branch from a harmful voltage dip, for example in the event of an abruptly rising power requirement, by connecting a powerful load in said second onboard electrical system branch. To this end, the onboard electrical system has an apparatus for voltage conversion that transforms the onboard electrical system voltage of the first onboard electrical system branch into the other onboard electrical system voltage of the second onboard electrical system branch.
Like many other electrical systems, the onboard electrical system of a vehicle is subject to a strict safety requirement to prevent, as far as possible, dangerous body currents that arise between two electrical connections at a contact voltage of above 60 V. To observe this safety requirement, the apparatus for voltage conversion for an onboard electrical system is normally equipped with transformers that provide DC isolation between the onboard electrical system branches. As a result of this DC isolation, no current can flow between the electrical connections of different onboard electrical system branches, namely between the connections on the input side and the connections on the output side of the apparatus. Therefore, there is also no voltage between the electrical connections of different onboard electrical system branches that could exceed the maximum permitted contact voltage. However, such apparatuses have a disadvantage in that they are expensive on account of the transformers with a DC isolation function.